1. Brief Explanation of Fourier Analysis as it is Relevant to the Current Invention
Fourier analysis has long been viewed as a useful tool in the use of electromagnetic waves, such as radio waves, to carry information. At a basic level, Fourier analysis represents an attempt to break down the composition of any type of wave into groups of pure harmonics which combine to produce the wave. As a brief example of the use of Fourier analysis, ocean waves may be observed as changes in water level at a fixed point over time. Yet, this water movement is also represented by a superposition of waves of distinct frequencies of oscillation, each wave of different frequency having a different strength. In other words, a non-periodic pattern is always representable as some combination of different harmonics at different strengths. Fourier analysis is a thus useful tool that enables conversion between time-based measurement, for example, water level as a function of time, and frequency-based measurement, or levels of harmonic strength as a function of frequency.
Fourier analysis has been applied significantly in recent times to speech and audio processing, and in particular to digital electronic audio processing systems. These digital systems, which for example include compact disk systems, utilize numbers in lieu of voltage levels which are used by more traditional analog electronics. As an example, audio information is frequently stored upon magnetic tape, yet the tape's storage conditions may affect the resolution of some audio data in playback. With digital systems, such as for example compact disk systems, individual numbers (which correspond to analog voltage levels at discrete, sequential times) may nearly always be exactly obtained and audio information derived therefrom. The only significant limitation is that sufficient quantities of numbers used to reproduce audio data need to be obtained to avoid an effect known as "aliasing." That is, the rate of numbers provided to reproduce audio data needs to be at least twice the highest frequency of the audio data.
In these digital systems, Fourier analysis is used to convert between a time-based measurement and a frequency-based measurement to enable analysis of harmonics. In many modern approaches, a process known as a Fast Fourier Transform may be used to convert time-based digital values into strength values for each of a number of separate frequencies that make up the time-based signal. The Fast Fourier Transform is simply a manipulation of time-based data, derived from traditional Fourier analysis of signals, which makes use of computational shortcuts, such as signal analysis over limited time ranges, frequency ranges, or other shortcuts. For example, since computers and other microprocessor-based systems typically perform many functions in a limited amount of time allocated for one program loop, they generally can perform only a small number of multiplications over localized digital values, to obtain current frequency data in relation to a small segment of the corresponding time-based signal. Thus, a Fast Fourier Transform implemented in computers and the like provide a tool whereby a limited number, generally a few dozen to one-thousand, of digital values that make up a digital signal which has been segmented in time are efficiently converted to time-localized, frequency-based information. The Fast Fourier Transform may be performed repeatedly, upon "windows" of the digital values, to efficiently derive time-localized frequency information over different segments of an audio signal.
2. The Problem at Hand in Relation to the Prior Art
One area of audio processing to which Fourier analysis has been applied is to the reduction of noise in analog audio signals. For example, a very simple type of noise reduction used for audio signals may be accomplished with an audio equalizer by attenuation of certain audio bands, reducing some types of hiss. However, this attenuation has the undesired effect of also attenuating wanted data of the audio signal.
One particular application of Fourier analysis to reduce noise is in hearing aids, where it is known to employ attenuation that is dependent upon the characteristics of the audio signal. For example, an especially high level of attenuation may be applied to frequency ranges that have no speech or other desired audio signal present which might be combined with noise. The attenuation may be lessened or removed when certain harmonics which represent desired audio signals are present.
Another application to which noise reduction schemes are important, and which motivates the preferred embodiment of the current invention, is in the recording of audio tracks, for example, speech or music. More particularly, the prior art does not provide low-cost noise reduction schemes which enable an operator to interactively modify attenuation in response to desired components of the audio tracks, such as speech, music, or other desired sounds that may be partially obscured by noise. Unlike the hearing aid systems, which are typically addressed to reducing ambient noise to sounds in the environment, noise reduction systems as applied to the generation of audio tracks preferably allow for operator control, as an operator may generally interactively distinguish unwanted obscuring noise from desired audio components and modify the noise reduction scheme in real-time. However, many of these noise reduction schemes are accomplished by use of an equalizer device, discussed above, which the operator may utilize to directly amplify or attenuate audio components which fall within distinct frequency bands. Equalizers are less than optimal, because to provide optimal noise reduction they must continuously and impractically be readjusted, for example, each time a particular sound or voice stops and starts.
A more popular noise reduction scheme used for restoration and registration of audio tracks which addresses this deficiency is a device commonly known as a "noise gate." A noise gate generally is an analog device that features a capacitive threshold that is applied across all frequencies that make up the desired audio signal. According to the attenuation scheme featured by typical noise gates, sounds that are loud are passed by the noise gate as an output signal with little or no attenuation, while softer sounds not meeting the electronically-imposed threshold are substantially attenuated.
Thus, using a rock concert as an example, a noise gate would generally attenuate constant sounds, such as crowd noise and the like. Some more advanced noise gates may feature as many as four different capacitive thresholds which are applied in isolation across different frequency ranges. However, these devices are not efficiently applied to restoration and registration of audio tracks. For example, using again the rock concert example, it might be desired to emphasize or de-emphasize crowd noise occurring within a single audio track over other sounds appearing within the same track. Alternatively, it might be desired to hear a cymbal crash, as an example, over a length of time, which might be not be passed by a noise gate as an audio output signal if the desired sound is long in duration. Furthermore, many of the advanced noise gates are prohibitively expensive.
There exists a need for a device that allows for real-time user-interaction to modify thresholds in response to perceived audio data. More particularly, there exists a need for a device that advantageously allows an operator to track and monitor background noise and manipulate attenuation in response thereto, preferably in independent fashion among numerous, distinct frequency bands. A need further exists for a device wherein a given frequency band may be automatically attenuated depending upon whether total sound within the frequency band falls below a threshold, but which doesn't require readjustment each time a voice, music or other desired sound starts and stops. Finally, there exists a need for a low-cost and efficient noise reduction system that can respond to operator control in real time and also to the frequency characteristics of input audio to which the noise reduction system is applied, preferably utilizing Fourier analysis to accomplish this object. The current invention satisfies these needs and provides further related advantages.